بررسي رفتار قطعات بتني غيرمسلح در برابر حريق

بتن يكي از مصالح پركاربرد در صنايع امروزي مي‌باشد كه نياز است رفتار آن در برابر پديده‌هاي محيطي و از جمله حريق مورد بررسي قرار گيرد. هنگامي كه قطعه‌ي بتني تحت افزايش دماي ناشي از حريق قرار مي‌گيرد، فشارحفره‌اي در آن تشكيل شده و اين حرارت منجر به ايجاد تنش‌هايي در بتن مي‌گردد كه با افزايش اين تنش‌ها، لايه‌اي از سطح بتن كنده مي‌شود و تورق صورت مي‌گيرد. در تحقيق پيش‌رو با ارائه‌ي يك مدل تحليلي برمبناي نرم‌افزار آباكوس، به بررسي رفتار قطعات بتني در برابر حريق پرداخته مي‌شود. در مدل مذكور پس از تحليل حرارتي اوليه، با ورود نفوذپذيري و ديگر مشخصه‌هاي بتن و نيز با الحاق سابروتين‌هاي مدنظر در تحليل ‘Soils’ از آباكوس، ميزان فشارحفره‌اي و تورق در بتن مشخص مي‌گردد. نتايج گوياي اين مهم است كه اعمال حريق هيدروكربني نسبت به حريق استاندارد، مي‌تواند بيش از دو برابر حداكثر فشارحفره‌اي بتن را افزايش دهد و روند تورق در بتن را تسريع كند و نيز با افزايش ده برابري نفوذپذيري بتن (از 5×10-17 m2 به 5×10-16 m2)، ميزان حداكثر فشارحفره‌اي بتن حين حريق استاندارد، به نزديك يكدهم تقليل يافته كه اين تغيير مي‌تواند منجر به جلوگيري از پديده‌ي تورق در قطعه‌ي بتني ‌گردد.

Concrete is a useful material in today industry that must be known behavior against environmental phenomenon including fire concurrent using this material in various industries. Generally, in concrete two phases being visible, the solid phase includes cement paste with other aggregates and additives, and liquid phase includes water placing in concrete pores. When applying the fire in the concrete segments, in addition to some reactions occur in the solid phase, some changes are seen in the liquid phase and gradually gas phase including vapor in the concrete pores will be built. Usually, evaporation starts in concrete pores water at the 100 °C temperature and since concrete temperature passes 150 °C all of the water in pores changes to vapor and flows in the colder side of concrete and accumulate at this portion. When 100 °C front temperature further progresses in the concrete, evaporation speed increases and flows in the colder side and accumulation zone of water grows until this zone becomes full of vapor and creates a saturated layer in pores that prevents from fluid flowing. After this layer saturated by vapor invasion from the hot side, pore pressure gradually develops and with this event tension stresses at this side of concrete will increase. With continue increasing fire intensity, saturated layer creation speed increases and pore pressure and tension stress increment occur. In the following of this process, if tension stress is becoming bigger than tensile strength, fracturing and loss of material from this layer are caused, i.e. spalling occurs. This paper present an analytical modeling using ABAQUS software to evaluating concrete fire behavior. Modeling and analysis of concrete slab under fire in this study includes some steps. In the first step, heat transfer modeling and analysis of solid part is done. In the following "soils" analysis based on initial heat transfer analysis result is accomplished, that vapor flowing between pores, the saturated layer forming and pore pressure developing occur in this step. Concurrent soils analysis, using USDFLD and UMESHMOTION subroutines that joined to software, pore pressure, and tension stress value are controlled and if spalling occurs in the concrete slab, spalled layer depth and time of spalling are determined and saved. One of the most important issues facing in the fire at structures is elevated temperature patterns subject and especially fire curves types. Because of the importance of this issue, some of these fire curves placed in the famous codes. One of the most popular fire curves in the structure is the ISO834 fire curve that based on cellulose fire. Although many researchers use the ISO834 fire curve in their research, when a fire occurs in structure with petroleum and hydrocarbons products, elevated temperature speed is higher than ISO834. So the fire curve based on petroleum product in the structure presented, is named hydrocarbon curve. Results show that applying Hydrocarbon fire in the concrete increase pore pressure more than double and accelerate spalling process in comparison to ISO834 fire and by modifying permeability of concrete from 5×10-17m2 to 5×10-16m2, pore pressure decrease less than one tenth even prevent spalling phenomenon.